It is known that a magnesium-containing catalyst for ethylene (co)polymerization shows significantly high catalytic activity, provides polymers with high bulk density, and is suitably used in liquid or gas phase polymerization. Liquid phase ethylene polymerization refers to a polymerization process carried out in solvent such as bulk ethylene, isopentane or hexane. The important features of catalyst used in such polymerization process, in view of its process applicability, are catalytic activity, bulk density of the resulting polymers, amount of low molecular weight polymers soluble in the medium, and the like. The molecular weight distribution in polymer varies depending on the properties of the catalyst per se and preparation conditions. For example, polymers obtained by using a Ziegler-Natta type catalyst in a single slurry or gas phase reactor generally have narrow molecular weight distribution. Owing to such feature of molecular structure, processability of the polymers is lowered and tensile strength and rigidity in a molten state is limited, thereby resulting in distortion and contraction of shape and Parrison deflection during processing. Moreover, these polymers are difficult to apply to the production of large caliber pipe or large blow molding products which require high mechanical resistance in a molten state. When raising the molecular weight of polymers produced, tensile strength is raised but processability is lowered, causing problems such as clefts and the like during processing. In order to overcome these problems, it is suggested to be an ideal method in view of molecular structure design to raise the molecular weight simultaneously with broadening the molecular weight distribution, since such method can maintain the tensile strength high and provide excellent processability.
Various titanium-based catalysts containing magnesium for olefin polymerization and various methods for producing them have been reported. In particular, a method using a magnesium solution to obtain an olefin polymerization catalyst which can provide polymers having high bulk density as mentioned above is widely known. With regard to this, a method for preparing a magnesium solution may be mentioned wherein magnesium compounds are reacted with an electron donor such as alcohol, amine, cyclic ether, or organic carboxylic acid in the presence of a hydrocarbon solvent. Methods using alcohol to prepare a magnesium solution are disclosed in U.S. Pat. Nos. 3,642,746, 4,336,360, 4,330,649 and 5,106,807. Well known is a preparation method for a magnesium supported catalyst in which the liquid magnesium solution is reacted with halogen compounds such as titanium tetrachloride. Although these catalysts may produce polymers having high bulk density, the catalysts need to be improved in catalytic activity and hydrogen reactivity. U.S. Pat. Nos. 4,477,639 and 4,518,706 disclose a method wherein tetrahydrofuran, a kind of cyclic ether, is used as a solvent for dissolving the magnesium compounds.
In addition, U.S. Pat. Nos. 4,847,227, 4,816,433, 4,829,037, 4,970,186 and 5,130,284 have reported a preparation method of olefin polymerization catalysts which have good catalytic activity and can provide resulting polymers having enhanced bulk density, by reacting electron donors such as dialkylphthalate or phthaloyl chloride with a titanium chloride compound.
U.S. Pat. No. 5,459,116 has reported a preparation method for a supported titanium solid catalyst by contacting a titanium compound with a magnesium solution containing esters having at least one hydroxyl group as an electron donor. By this method, it is possible to obtain a catalyst which has high catalytic activity and can provide resulting polymers having high bulk density, but it still has to be improved in terms of molecular weight distribution.
U.S. Pat. No. 3,899,477 discloses a catalyst for which titanium halide, vanadium halide and organoaluminum compounds are used together. When treating this catalyst with alkylaluminum sesquiethoxide and trialkylaluminum before polymerization, it is possible to produce polymers having broad molecular weight distribution. This prior art, however, is disadvantageous in that the catalyst preparation process is complicated and it is difficult to control the condition of polymerization process due to the differences in hydrogen reactivity between titanium and vanadium, and in reactivity to monomers and co-monomers.
As it has been shown above, for a conventional Ziegler-Natta type catalyst, it is difficult to have high catalytic activity and control the molecular weight distribution of resulting polymers. However, there is still a demand for a method for controlling the molecular weight distribution, in order to further control the processability and properties of ethylene polymers and extend their use.
The applicant of the present invention, therefore intends to propose a novel method for ethylene (co)polymerization with high catalytic activity and being able to control the molecular weight distribution.